Conventional control circuits for d.c. motors are utilized in many industrial machines such as paper processing apparatus, loaders, trucks and the like. These known circuits usually employ silicon controlled rectifiers integrated in the motor electric supply circuit, and some device to produce a train of pulses arranged to effect periodic conduction of the rectifiers. In this manner, the motor is energized by pulses of direct current, and control of the duration of the pulses is utilized for motor control purposes such as speed of the motor drive, and for compensation during various load conditions.
It has been found that more precise control of motor speed is available if compensation is considered, and provision made for the effect of the internal resistance in motor armatures. Such internal resistances produce corresponding voltage drops across the armature in accordance with the current to the armature. Since motor speed is proportional to the voltage applied to the motor, any voltage drop across the armature will correspondingly lower the voltage available for energization of the motor.
In order to offset the controlling effects resulting from this inherent characteristic, use is made of compensating circuits which are adapted to adjust, or to cancel out, the effect of armature resistance.
One method of compensation in this regard is to boost the voltage produced in the control circuit, and made available for the motor supply, to an amount equal to the voltage drop at any controlled motor speed.
In U.S. Pat. No. 3,551,774, a circuit is disclosed for controlling current to a d.c. motor utilizing temperature variations in the rectifier circuit, which serves as the electrical power supply for the d.c. motor. While this arrangement serves as a good safety technique for the motor and the rectifier circuit, accurate control of motor speed, and responsiveness to the need for quick control, are significantly reduced.
In U.S. Pat. No. 4,514,665, a more sophisticated current limit control is described, including a microprocessor for controlling appropriate pulse train waveforms for the controlling of speed to a vehicle d.c. motor. The circuitry and components therefor to accomplish this control function are rather involved and costly. Because of the indirect application of a control signal, it is doubtful that the response time for motor reaction is sufficiently fast for operator use. In any event, the required components are not necessarily inexpensive when considering invested capital.
In conventional control circuits, current is sensed across the armature of the d.c. motor by utilizing a resistor specifically assigned for this sensing purpose. The voltage developed across this resistor is amplified, compared to a reference voltage and then applied, perhaps through modifying circuits such as for current compensation, to the power supply of the motor being controlled. A trigger circuit may be utilized for the direct application of an error signal produced as a result of the comparison of the sensed voltage, thereby effecting appropriate corrective control of the motor.
Conventional controlling circuits, however, in amplifying current limiting signals, utilize amplifier circuits which perform other amplifying functions, such as in producing the compensating current signal for the cancelling out of the effect of the armature resistance as discussed above. In control circuits on the market today, a single amplifier is utilized with a set of voltage dividers. One divider is used for the current limiting function, while the other divider is used for current/resistance compensation. The disadvantage in this arrangement is that response to the production and application of corrective current limiting signals is slow, since the time response for compensation is itself slow. While the time constant involving the current limiting function is much faster, nevertheless, the time constant for the amplifier must be made as slow as the slowest required signal.